When multiple animations are meant to be executed in "parallel",
currently they will be asynchronous and thus experience an delayed
animation for later elements. To overcome this issue, we add an slot
mechanism which specifies that we can use one single slot for a bunch
of animations, and they all will use the same start time and current
time.

Would it be possible to have a callback added for when all elements in a particular slot have finished execution? Also I find the terminology of "slot" to be a little odd, wouldn't "set" or "group", possibly prefixed like "fxgroup" or "AnimationSet" be more appropriate?